Lithium-ion secondary batteries are widely used as rechargeable power sources for many applications such as consumer electronics, electric vehicles, hybrid electric vehicles (HEVs), satellites, spaceships, medical devices, and power tools. For such applications, Li-ion secondary batteries exhibiting higher energy densities and better thermal safety are desired when compared to those Li-ion cells that are currently available.
In general, the energy density and thermal safety of a Li-ion cell depends on the cathode material used in the cell. To increase the energy density of a Li-ion cell, it is necessary to develop cathode materials which have a high practical capacity and high material density to increase gravimetric and volumetric capacity, respectively. Cathode-related thermal safety problems may be caused by reactions between cathode particle surfaces and the organic electrolytes in charged states. Therefore, to improve the thermal safety of Li-ion cells, cathode materials must be developed with inherent thermal safety and/or which have a surface area that is as small as possible. Also, cathode materials with smooth surface morphologies can enhance the thermal safety characteristics of the cells. Improved safety results from smooth surfaces because the reactivity of materials with an organic electrolyte is very high at sharp edges due to very high chemical activity of ions at interfaces with high curvature.
In general, the morphology of calcined metal oxides is determined by the starting metal precursors and the synthetic methods employed. Both of these considerations also play an important role in controlling the electrochemical properties of the cathode materials in Li-ion cells. Co-precipitation of mixed metal hydroxides is the most widely adopted process to prepare dense, spherical metal precursors. However, co-precipitation of mixed metal hydroxides requires careful control of certain experimental parameters, such as pH and atmosphere. Such control is especially important when the mixed metal hydroxides contain manganese, due to the instability of Mn(OH)2. (J. Ying et al., J. Power Sources, 99, 78 (2001); M. H. Lee et al., Electrochim. Acta, 50, 939 (2004)). When manganese is used as one of the major constituents of the co-precipitated hydroxides, the co-precipitated hydroxide particles do not form dense, spherical shapes. However, since manganese-based cathode materials are promising for other reasons, methods of making dense spherical manganese oxides are highly desirable. More generally, there is a need in the art for inexpensive and operatively simple methods of forming dense spherical particles of active materials for lithium ion cells.